Acid mine drainage occurrence and its remediation

ACID MINE DRAINAGE

Refers to the flow of water out of a mine that has a very high acidic(low pH) after being in contact with air and metal.

Acid mine drainage occurrence

• Acid Mine Drainage results from the oxidation of sulfide minerals inherent in some ore bodies and the surrounding rocks.

• Iron sulfide minerals, especially pyrite (FeS2), chalcopyrite (FeS.CuS) and also pyrrhotine(FeS) contribute the most to formation of Acid Mine Drainage.

• Oxygen (from air or dissolved oxygen) and water (as vapor or liquid) which contact the sulfide minerals directly cause chemical oxidation reactions which result in the production of sulfuric acid.


• Overall chemical reactions associated with pyrites can be described as follows;

- Pyrite is initially oxidized by atmospheric oxygen producing sulfuric acid and ferrous iron (Fe2+) according to the following reaction:

- FeS2 + 7/2 O2 + H2O > Fe2+ + 2SO42- + 2H+…….. (1)

Conversion of ferrous iron to ferric iron.

- Fe2+ + 1/4 O2 + H+ > Fe3+ + ½ H2O .........(2)


• Ferrous iron may be further oxidized by oxygen releasing more acid into the environment and precipitating ferric hydroxide.

- Fe2+ + 1/4 O2 + 5/2 H2O > Fe (OH) 3 + 2H+........(3)

• As acid production increases and the pH drops (to less than 4), oxidation of pyrite by ferric iron(Fe3+) becomes the main mechanism for acid production

- FeS2 + 14Fe3+ + 8H2O > 15Fe2+ + 2SO42-+ 16H+……. (4)

• This reaction is catalyzed by the presence of Thiobacillusferrooxidans which accelerates the oxidation of ferrous iron into ferric iron (reaction 2) by a factor of 106:1.


• The sulfuric acid produced in these reactions increases the solubility of other sulfide minerals in the solid surfaces. Ferric iron in acidic solution can oxidize metal sulfides per the following reaction:

• MS + 2Fe3+ > M2+ + S + 2Fe2+ …………………………..(5)

Where MS = metal sulfide (galena, sphalerite, etc.)


• Metals commonly solubilized from sulfides in AMD include aluminum, copper, lead, manganese, nickel, and zinc. Metals in the form of carbonates, oxides, and silicates may also be mobilized, often aided by biological catalysts.

• AMD may also leach uranium, thorium, and radium from mine wastes and tailings associated with uranium mining operations

AMD effects on environment

• AMD harms the environment by lowering soil and water pH hence increase heavy metals drainage from rocks and soil colloid materials into water bodies and the environment in general

• At low pH levels heavy metals release from soil colloids increase

• Accumulation of heavy metals in soil and water leads to increase bio-concentration and bio-accumulation in plants, fish, livestock and humans through the process of eating and being eaten.

AMD Prevention and Mitigation

• Minimizing oxygen supply because of diffusion or advection

• Minimizing water infiltration and leaching (water acts as both a reactant and a transport mechanism)

• Minimizing, removing, or isolating sulphide minerals

• Controlling pore water solution pH

• Maximizing availability of acid neutralizing minerals and pore water alkalinity

• Controlling bacteria and biogeochemical processes

AMD Control Strategies

• Containment and isolation

- Soil covers- by imported materials e.g. clay, soil

- low sulphide waste-rock if compactable

- Geo-textile fabrics

- Water cover- creation of a permanent lake or swamp

- use of an existing lake

- flooding of underground tunnels

- submarine disposal

-Blending- mixing of acid and non acid forming waste rock

• Treatment- Using AMD remediation technologies


Soil Covers

• Soil covers generally involve the use of granular earthen materials placed over mine wastes. The objectives of a soil cover varies from site to site, but generally include (i) dust and erosion control; (ii) chemical stabilization of acid-generating mine waste (through control of oxygen or water ingress); (iii) contaminant release control (through improved quality of runoff water and control of infiltration); and/or (iv) provision of a growth medium for establishment of sustainable vegetation


Key factors to consider in the design of a soil cover include

• The climate regime at the site

• The reactivity and texture of the mine waste material

• The geotechnical, hydrologic, and durability properties of economically available cover materials

• The hydrogeologic setting of the waste storage facility

• Long-term erosion, weathering, and evolution of the cover system


Processes Affecting Long-Term Performance of Soil Covers


Limitations of soil covers• Soil covers do not stop infiltration and may not stop acid

drainage.• Permeability of water infiltration barriers may increase with

time when subjected to climate and vegetation.• Oxygen barrier covers are especially vulnerable to relatively

small imperfections in the cover – such as differential settlement, holes caused by animal burrows, desiccation cracking etc.

• Soil covers may be prone to erosion and long-term maintenance requirements

• Soil covers may be vulnerable to vegetation, animal, and human activity including vehicle traffic

AMD Remediation Technologies

• Abiotic remediation

- Active technologies

- Passive technologies

• Biological remediation

- Passive technologies


Active remediation technologies

• One of the most common ways used is chemical precipitation

• These systems usually include:

1. equipment for introducing the neutralizing agent used to treat the Acid Mine Drainage 2. means for mixing the two streams 3. procedures for ensuring iron oxidation 4. settling ponds for removing iron, manganese, and other co-precipitates


Passive remediation technologies

• Require little to no maintenance

• Typically in the form of a wetland

• Cuts down on the price immensely

• However must remove the buildup of metal accumulations

• Very common method used because of its ability to cut down on price and the utilization of nature and not chemicals to remediate the water

Abiotic remediation

Active technologies

• The most widespread method used to mitigate acidic effluents is an active treatment process involving addition of a chemical-neutralizing agent.

• Addition of an alkaline material to AMD will raise its pH, accelerate the rate of chemical oxidation of ferrous iron (for which active aeration, or addition of a chemical oxidizing agent such as hydrogen peroxide, is also necessary), and cause many of the metals present in solution to precipitate as hydroxides and carbonates

Abiotic remediation

Active technologies cont…

• Various neutralizing reagents have been used, including lime (calcium oxide), slaked lime, calcium carbonate, sodium carbonate, sodium hydroxide, and magnesium oxide and hydroxide.

• Neutralizing agents vary in cost and effectiveness; for example, sodium hydroxide is some 1.5 times as effective but is about nine times the cost of lime.

Abiotic remediation

Passive technologies- Anoxic limestone drains

• An alternative approach for addition of alkalinity to AMD is the use of anoxic limestone drains (ALD).

• The objective of the systems is to add alkali to AMD while maintaining the iron in its reduced form to avoid the oxidation of ferrous iron and precipitation of ferric hydroxide on the limestone), which otherwise severely reduces the effectiveness of the neutralizing agent.

• In an ALD, mine water is constrained to flow through a bed of limestone gravel held within a drain that is impervious to both air and water (generally constructed of a plastic bottom liner and a clay cover).

Abiotic remediation

Anoxic Limestone drains

Abiotic remediation

Cross-section of an ALD

Abiotic remediation

Open Limestone Channels

• Open limestone channels are long channels or ditches lined with limestone to increase the alkalinity of the water.

• The acid water flows down the limestone bed and AMD is treated by\limestone dissolution.

• The construction of an OLC is determined by the flow rate, channel slope, and influent acidity concentration and this information will dictate the weight of limestone, the cross-sectional area and the length of the channel, and the in-channel retention

Abiotic remediation

Open limestone channel cont…

• One of the primary design factors is to have a steep enough slope and large enough limestone particle size to prevent iron and aluminum hydroxides from plugging up limestone pores.

• The ideal limestone size is 15-30cm in diameter.

• Another important factor when designing an OLC is the residence time. The quantity of time that the limestone is in contact with the AMD water is central and is a function of influent acidity and flow. Residence time of the AMD water with limestone is calculated as;

R.T (hours) = Length (ft). * Width (ft.) * Depth (ft)* Void Ratio (%) / Flow (cfs) * 3600

Abiotic remediation

An example of an open limestone channel

Biological remediation

• Passive biological systems- Constructed wetlands

• Constructed wetlands utilize soil- and water-borne microbes associated with wetland plants to remove dissolved metals from mine drainage.

• Aerobic wetlands are generally constructed to treat mine waters that are net alkaline.

• This is because the main remediative reaction that occurs within them is the oxidation of ferrous iron and subsequent hydrolysis of the ferric iron produced, which is a net acid generating reaction (Eqs. (1) and (2)).

- 4Fe2+ + O2 + 4H+> 4Fe3+ + 2H2O........(1)

- 4Fe3++ 12H2O> 4Fe (OH) 3 + 12H+ ....(2)


Constructed wetland cont…

• If there is insufficient alkalinity in the mine water to prevent a significant fall in pH as a result of these reactions, this may be amended by the incorporation of, for example, an anoxic limestone drain.

• In order to maintain oxidizing conditions, aerobic wetlands are relatively shallow systems that operate by surface flow.

• An aerobic wetland comprises channels or basins with an impermeable bottom (to limit seepage loss), a medium to sustain vegetation, and a shallow water depth (10-50cm) in order to allow water to contact the soil medium



• Anaerobic wetlands are designed to treat net acidic and metal-laden waters.

• They can be used only if a large enough area of land is available.

• Anaerobic wetlands usually consist of a limestone layer overlain by an organic compost layer, over which the AMD flows.

• Water quality in wetlands is improved by filtration of suspended and colloidal materials, and by adsorption of metals to the soil substrate or other organic-based substrates.


Constructed Wetland



• Constructed wetlands are efficient when the effluent is low in its acidity loading, but often show reduced efficiency and even fail under high acid loading. For this reason, alkalinity additions to wetland influent may precede constructed wetland.


Sulfate Reducing Bioreactors

• Sulfate reduction has been shown to effectively treat AMD/ARD containing dissolved heavy metals, including aluminum, in a variety of situations. The chemical reactions are facilitated by the bacteria desulfovibrio in sulfate-reducing bioreactors

• The sulfate-reducing bacterial reactions involve the generation of:

- Sulfide ion (S-2), which combines with dissolved metals to precipitate sulfides

- Bicarbonate (HCO3), which has been shown to raise the pH of the effluent

Scheme for passive treatment of AMD

Flowchart for selecting a passive AMD treatment system

AMD from waste rock piles at Nyabigena North Mara- TZ

Acid mine drainage occurrence and its remediation

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